Local coastal impacts underestimated from sea-level rise

The Mississippi Delta in 2001, imaged by NASA's Terra satellite.

Credit:

NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team

Most studies indicate that sea levels will rise over the next century due to melting glaciers, more ice breaking off the Antarctic ice sheet and thermal expansion — and there is great variation in how much scientists estimate seas will rise. But that’s not even the most important question, according to a new study. Instead, researchers should be looking at relative sea-level rise — how much rising seas will affect individual regions. And when you break it down by region, the study suggests, the outlook isn’t promising.

One of the regions likely to be most affected by even a small rise in sea levels is the Gulf Coast of the United States, reported John Anderson, a geologist at Rice University in Houston, Texas, and his colleagues in a recent issue of EOS. When it comes to relative sea-level rise, even if the rise is just a couple of millimeters — at the lower end of the projected sea-level rise over the next century — the Mississippi Delta is in danger, Anderson says. Anderson calculated the region’s relative sea-level rise rates from data from the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report. And some reports, including IPCC, suggest that sea levels could rise by 60 centimeters by 2100.

Over the past few centuries, some 25 percent of the coastal wetlands of Louisiana and Mississippi have been claimed by rising seas. In the Gulf Coast, sea-level rise will be worse than other areas, because the Mississippi River has been lined with levees to avoid flooding of populated areas, which prevents the river from depositing enough sediment to counteract rising sea level. In a natural system, when the river floods swamps and marshes each spring, it deposits sediment in wetlands, which helps build up the land. Because this isn’t happening, huge tracts of land and valuable swamps and marshes are sinking and could be flooded by the encroaching sea, Anderson says.

Even along the Gulf Coast, however, Anderson says, every beach will be affected differently. How a beach will be affected largely depends on the type of sand on the beach and how much sediment local rivers are depositing to counteract the sea-level rise. But in general, Anderson says, the rates of relative sea-level rise are grossly underestimated, primarily because scientists are unsure of the rates of melting in Greenland and Antarctica and because local effects are not well-studied. Problems connected with the rising seas will not be solved until researchers consider other factors, such as relative rates of sea-level rise, he adds.